This trouble is coupled with the fact that I really dont have a great concept of how 3D games work, especially the concept of coordinet spaces, but after some research here is what i think i figured out.
my first mistake was implementining entities in a totaly different space than the terrain. this is problematic since the entities need to move with the terrain.
so instead the entity billboards need to exist in model space, be transformed into world space and then into view space, and projected orthographically.
i miss the ease of a single 2D coord space =/